1. Field of the Invention
The present invention relates to a mounting board on which an electronic component can be flip-chip mounted using bump electrodes, and to an electronic device using the mounting board.
2. Description of the Related Art
In known electronic devices, an electronic component, such as a semiconductor element, is flip-chip mounted on a circuit board using bump electrodes. Such electronic devices are formed by mounting the electronic component having bumps face-down on connection electrodes formed on the circuit board, filling a gap between the circuit board and the electronic component with sealing resin, and heat-hardening the sealing resin in an oven. The sealing resin between the circuit board and the electronic component contributes to mitigation of thermal stress applied to the bumps due to the difference in thermal expansion coefficient between the electronic component and the circuit board.
In such electronic devices, for example when solder bumps are used as the bump electrodes, an insulation film is formed over wiring electrodes to prevent the solder from spreading (wetting) more than necessary. The insulation film is also useful, for example, for covering an appropriate portion of metal plating, such as Au, or in order to prevent the wiring electrodes from being stripped off from the circuit board.
FIGS. 8 and 9 show the structure of an electronic device of the related art.
An insulation film 31 is formed on substantially the entirety of a surface of a circuit board 30. The insulation film 31 has opening portions or uncoated portions 31a in a one-to-one relation with wiring electrodes 32. Each of the opening portions 31a is slightly wider than the corresponding wiring electrode 32 in order to improve the positional accuracy. Small recesses 31b each having a depth equal to the thickness of the insulation film 31 are formed at both sides of each wiring electrode 32. The insulation film 31 is typically formed by printing solder resist, and the thickness of the insulation film 31 is about 30 μm to 50 μm.
An electronic component 33 is flip-chip mounted on the mounting board 30 by connecting bump electrodes 34 with portions of the wiring electrodes 32 exposed at the opening portions 31a. When sealing resin 35 is filled in a gap between the circuit board 30 and the electronic component 33, the sealing resin 35 flows through the gap and over the insulation film 31 by capillary action. However, the recesses 31b have a small volume, thus causing air bubbles 36 to be trapped. The trapped air bubbles 36 cause voids to be generated in the hardened sealing resin 35, thus reducing its reliability.
As proposed in Japanese Unexamined Patent Application Publication No. 8-153752, one method to overcome such a problem is to insert the sealing resin and then defoam or de-air the sealing resin by applying a vacuum or by applying ultrasonic vibration.
However, such a de-airing process is not always effective to remove the trapped air, and voids may still be caused.